1. Field of the Invention
The invention relates to a method of producing flat, transparent vitreous silica bodies of optical quality and low bubble content, in which granules or powder ground from crystalline quartz, amorphous vitreous silica or pure quartz sand as starting material, is melted in a crucible under vacuum by means of an electrical heating means, and an apparatus for carrying out such method.
2. Discussion of Prior Art
German Pat. No. 445,763 discloses a method of producing flat objects from fused quartz by means of electrical resistance furnaces. In this method, a rod-like electrical heating resistance is surrounded by the starting material to be melted and, and the action of the heating current, produces a fused tubular body. Then the heating resistance is rapidly drawn out of it, and the body is squeezed flat to form a plate.
German Pat. No. 697,699 discloses the production of bubble-free bodies from fused vitreous silica. The starting material is mostly clear pieces of natural quartz crystal, which are melted down and then shaped by conventional methods. It also discloses the production of vitreous silica bodies by first forming compacts of the desired shape from silica powder and then heating them in a vacuum in an electrical furnace until they vitrify. The production of quartz blocks in a vacuum from granular starting material is also known, the blocks being then reheated and pressed to final shape.
In view of the fact that the products produced by the above-mentioned methods contain a great many bubbles, even though German Pat. No. 697,699 speaks of bubble-free bodies, the production of substantially bubble-free, clear vitreous silica products, i.e., products of low bubble content, has been accomplished in accordance with U.S. Pat. No. 2,726,487 by impregnating the granular starting material, such as pure quartz sand, with a silicate solution and, after drying out the liquid component, causing SiO.sub.2 to become deposited in the pores of the starting material, the material is melted in a vacuum in a graphite crucible. In this manner, transparent, flat slabs of low bubble content are obtained from vitreous silica.